Sinub

I wanted to know why you have to swap transmissions sometimes while engine swapping especially on FWD engine layouts to RWD layouts. I'm pretty sure it doesn't have to specifically do with which way the engine spins, clockwise or counter clockwise. I know the D, B, series spin counter clockwise. K series engines spin clockwise though some people have swapped it into the s2k( which that engine also spins the same way as the K series I believe) Why do people use different transmissions?

What are the reasons?

Why can't you use the old transmissions and what would you have to do if you do swap?